As semiconductor devices reach ever smaller dimensions, the challenge of power dissipation and quantum effect place a serious limit on the future device scaling. Recently, a multiferroic tunnel junction (MFTJ) with a ferroelectric barrier sandwiched between two ferromagneticelectrodes has drawn enormous interest due to its potential applications not only in multi-level data storage but also in electric field controlled spintronics and nanoferronics. Here, we present our investigations on four-level resistance states, giant tunneling electroresistance (TER) due to interfacial magnetoelectric coupling, and ferroelectric control of spin polarized tunneling in MFTJs. Coexistence of large tunneling magnetoresistance and TER has been observed in manganite/(Ba, Sr)TiO3/manganite MFTJs at low temperatures and room temperature four-resistance state devices were also obtained. To enhance the TER for potential logic operation with a magnetic memory, La0.7Sr0.3MnO3/BaTiO3/La0.5Ca0.5MnO3 /La0.7Sr0.3MnO3 MFTJs were designed by utilizing a bilayer tunneling barrier in which BaTiO3 is ferroelectric and La0.5Ca0.5MnO3 is close to ferromagnetic metal to antiferromagnetic insulator phase transition. The phase transition occurs when the ferroelectricpolarization is reversed, resulting in an increase of TER by two orders of magnitude. Tunneling magnetoresistance can also be controlled by the ferroelectricpolarization reversal, indicating strong magnetoelectric coupling at the interface.

Hitperm-type rapidly quenched ribbons were submitted to field annealing, both longitudinal field (LF) and transversal field (TF) to the axis of the ribbon. LF annealing yields a reduction of the magnetic anisotropy and results can be explained in the frame of random anisotropy model. A coercivity of 3 A/m is obtained for Fe39Co39Nb6B15Cu1alloy. The addition of Cu to these Nb-containing Hitperm-type alloys is a key factor to refine the microstructure in order to reach this very low coercivity value. TF annealing produces samples with sheared hysteresis loops suitable for sensor and high frequency applications.

Magnetoresistance(MR) reported in some non-magnetic semiconductors (particularly silicon) has triggered considerable interest owing to the large magnitude of the effect. Here, we showed that MR in lightly doped n-Si can be significantly enhanced by introducing two diodes and proper design of the carrier path [Wan, Nature 477, 304 (2011)]. We designed a geometrical enhanced magnetoresistance (GEMR) device whose room-temperature MR ratio reaching 30% at 0.065 T and 20 000% at 1.2 T, respectively, approaching the performance of commercial MR devices. The mechanism of this GEMR is: the diodes help to define a high resistive state (HRS) and a low resistive state (LRS) in device by their openness and closeness, respectively. The ratio of apparent resistance between HRS and LRS is determined by geometry of silicon wafer and electrodes.Magnetic field could induce a transition from LRS to HRS by reshaping potential and current distribution among silicon wafer, resulting in a giant enhancement of intrinsic MR. We expect that this GEMR could be also realized in other semiconductors. The combination of high sensitivity to low magnetic fields and large high-field response should make this device concept attractive to the magnetic field sensing industry. Moreover, because this MR device is based on a conventional silicon/semiconductor platform, it should be possible to integrate this MR device with existing silicon/semiconductor devices and so aid the development of silicon/semiconductor-based magnetoelectronics. Also combining MR devices and semiconducting devices in a single Si/semiconductor chip may lead to some novel devices with hybrid function, such as electric-magnetic-photonic properties. Our work demonstrates that the charge property of semiconductor can be used in the magnetic sensing industry, where the spin properties of magnetic materials play a role traditionally.

Magnetostrictive behaviors under rotating magnetic fields are investigated for bcc(001) single-crystalfilms of Fe100−x-Six(x = 0, 6, 10 at. %). The magnetostriction observation directions are along bcc[100] and bcc[110] of the films. The magnetostriction waveform varies greatly depending on the observation direction. For the observation along [100], the magnetostriction waveform of all the films is bathtub-like and the amplitude stays at almost constant even when the magnetic field is increased up to the anisotropy field. On the other hand, the waveform along [110] is triangular and the amplitude increases with increasing magnetic field up to the anisotropy field and then saturates. In addition, the waveform of Fe90Si10film is distorted triangular when the applied magnetic fields are less than its anisotropy field. These magnetostrictive behaviors under rotating magnetic fields are well explained by employing a proposed modified coherent rotation model where the anisotropy field and the magnetization reversal field are determined by using measured magnetization curves. The results show that magnetocrystalline anisotropy plays important role on magnetostrictive behavior under rotating magnetic fields.

Wireless sensors capable of scavenging energy from ambient environment have been increasingly attractive for their outstanding merits of self-sufficient and maintenance-free. This paper presents a specific design of magnetic energy harvester based on a piezoelectric/magnet composite and a magnetic concentrator. With the employment of concentrator, the energy harvesting properties have been greatly improved, which is theoretically analyzed and experimentally demonstrated with the 35 times power enlargement. The fabricated prototype with a 3 cm air-gap concentrator harvests 326 μW power at 10 Arms, which enables sufficient and reliable power supply for a wide range of low-power sensors.

Techniques for identifying defects in metals are very important in a wide variety of manufacturing areas. The present paper reports an eddy current testing method that employs a nano-granular in-gap magnetic sensor (GIGS) to detect internal defects in aluminum boards. The GIGS consists of a tunnel magnetoresistive film with nanometer sized grains and two yokes. In the presence of an external magnetic field, the nano-granular film exhibits only a small change in resistance due to the tunnel magnetoresistive effect. However, by placing it between two yokes, the magnetic flux can be greatly concentrated, thus increasing the change in resistance. The GIGS is a magnetic-field sensor that exploits this principle to achieve enhanced sensitivity. Moreover, because it has a cross-sectional yolk area of just 80 μm × 0.5 μm, it achieves outstanding spatial resolution. In the present study, it is used in combination with an eddy-current method in order to detect internal defects in aluminum. In this method, an excitation coil is used to apply an AC magnetic field perpendicular to the aluminum surface. This induces eddy currents in the metal, which in turn give rise to an AC magnetic field, which is then measured by the GIGS. The presence of defects in the aluminum distorts the eddy current flow, causing a change in the magnitude and distribution of the magnetic field. Such changes can be detected using the GIGS. In the present study, the proposed method was used to successfully detect indentations with diameters of 5 mm on the rear surface of an aluminum plate.

Low magnetic loss ferrite composites consisting of Ba(CoTi)1.2Fe9.6O19 and BiFeO3 (BFO) ferrite were investigated for permeability, permittivity, and high frequency losses at 10 MHz–1 GHz. The phase fraction of BiFeO3 was quantitatively analyzed by X-ray diffractionmeasurements. An effective medium approach was employed to predict the effective permeability and permittivity for the ferrite composites, which was found to be in good agreement with experimental data. The experiment demonstrated low magnetic losses (<0.128), modified by BFO phase fraction, while retaining high permeability (∼10.86) at 300 MHz. More importantly, the BFO phase resulted in a reduction of magnetic loss by 32%, as BFO phase increased from 2.7 vol. % to 12.6 vol. %. The effect of BFO phase on magnetic and dielectric properties revealed great potential for use in the miniaturization of high efficiency antennas.

The application of giant magnetoresistance(GMR)currentsensors in power grid and other industrial fields has a great prospect benefitting from their wide bands, high sensitivity, and good linearity. This paper studies the influence of mobile ions on current measurement of GMRsensor under high external electric field. The R-H curves of GMR multilayer sensor were depicted when the external electric and magnetic fields were both changed under three orthogonal electric field orientations. The experiment results indicate slightly varying resistances of GMRsensor when the external electric field was changed, and simulation analysis reveals that the resistance variation is attributed to the movement of surface ions under high external electric field. Therefore, a Faraday box is essential for GMRsensors to avoid interferences under high-strength field conditions, which is especially significant for their application as currentsensors of the power grid.

Metallic implants can result in considerable inhomogeneity in the signal intensity of magnetic resonance imaging(MRI), because the implant generates a shielding effect to the applied radio-frequency (RF) magnetic fields. In this study, we propose an acquisition method to mitigate the signal inhomogeneities using an adaptive RF pulse waveform. The effectiveness of the method was investigated using both numerical simulations and experiments. The RF pulse waveform was calculated based on inverse analyses of the Bloch equation incorporating the measured RF field distribution within the object. A simulation was carried out using a simplified numerical model of RF field inhomogeneity assumed at the center of model. An RF pulse waveform was designed to recover the attenuated signal region in the given model, and we show a significant improvement in the signal homogeneity compared with that obtained using a conventional pulse. We implemented the proposed method on a 7T-MRI system to show the efficacy experimentally. Test samples were fabricated from agarose gel with inserted copper or aluminum implants of different thicknesses. The RF pulse for selective excitation was calculated after mapping the RF field distribution of each imaging object. The acquired images exhibit an improvement in the homogeneity at the region of metallic implants. These results indicate that the proposed method is effective for MRI measurements of objects containing metallic implants.

The magneto-dielectriceffect (MDE) involves studies on the influence of an applied magnetic field on the dielectric constant of a material. MDEs in self-assembled core-shell nanoparticles of nickelferrite and barium titanate have been investigated in the millimeter wave frequencies. The core-shell nanocomposites were synthesized by coating 15 nm nickelferrite and 100 nm barium titanate nanoparticles with complementary coupling groups and allowing them to self-assemble in the presence of a catalyst forming heterogeneous nanocomposites. Studies on MDE in as-assembled particles have been carried out by measurements of the relative permittivity as a function of frequency f under an applied static magnetic field H over 16–24 GHz. Measurements show an H-induced decrease in permittivity by 0.8% for H = 4 kOe and is much stronger than MDE in single phase multiferroics. A model for the high frequency MDE has been discussed here.

Temperature-dependent magnetotransport properties of the antiferromagnetic semiconductor Sr2IrO4 are investigated with point-contactdevices. The point-contact technique allows to probe very small volumes and, therefore, to look for electronic transport on a microscopic scale. Point-contactmeasurements with single crystals of Sr2IrO4 were intended to see whether the additional local resistance associated with a small contact area between a sharpened Cu tip and the antiferromagnet shows magnetoresistance(MR) such as that seen in bulk crystals. Point-contactmeasurements at liquid nitrogen temperature revealed large MRs (up to 28%) for modest magnetic fields (250 mT) applied within an IrO2 (ab) plane with angular dependence showing a crossover from four-fold to two-fold symmetry with an increasing magnetic field.Point contactmeasurement exhibits distinctive anisotropic magnetoresistance(AMR) in comparison to a bulk experiment, imposing intriguing questions about the mechanism of AMR in this material. Temperature-dependent MRmeasurements show that the MR falls to zero at the Neel temperature, but the temperature dependence of the MR ratio differs qualitatively from that of the resistivity. This AMR study helps to unveil the entanglement between electronic transport and magnetism in Sr2IrO4 while the observed magnetoresistive phenomena can be potentially used to sense the antiferromagnetic order parameter in spintronic applications.

We investigate the microwave absorbing characteristics in grid-shaped rubbercomposite sheets containing Ag-coated Ni-Zn ferrite particles. The improvements in the microwave absorbance are obtained through the insertion of a periodic air cavity in the high-permittivity composites. In the bulk specimens containing conductive and magnetic Ag-coated ferrite microspheres, the impedance matching is not satisfied due to the high dielectric permittivity of the composite. The insertion of an air cavity in those absorbers reduces the permittivity and permeability, and thereby leading to impedance matching at a higher frequency. In the grid-type absorber with an optimum air cavity volume rate, the reflection loss can be decreased to as low as −30 dB at 10.5 GHz with a small layer thickness of 2 mm. The proposed grid-type microwave absorber has advantages of a small matching thickness and a considerable reduction in weight in comparison with conventional ferritecomposite absorbers.

A detailed comparative Ferromagnetic resonance study of pulsed laser deposited
Co40Fe40B20thin films,
before and after annealing, was under taken. The dependence of resonance field
(Hres) and peak-to-peak linewidth
(ΔHpp) on film thickness, annealing temperature, and magnetic field
orientation is examined. ‘In-plane’ (IP) and ‘out-of-plane’
(OP) angular dependence of the resonance fields,
(IP:Hres(ψ);
OP:Hres(α)), were measured at T = 150 and
295 K for the as deposited (as-) to annealed (an-) thin film samples to
determine IP and OP uniaxial anisotropy fields. Variation of
Hres(ψ) and
Hres(α) on sample geometry demonstrate that
the uniaxial magnetic
anisotropy is present in as- and an-thin films of
Co40Fe40B20.
The effective magnetic
anisotropy increases after nanocrystallization in CoFeB films indicates that the
exchange
interactions are unable to average out the local-magnetocrystalline
anisotropy of the nanocrystalline grains and thereby lead to magnetic
hardening in the early stages of crystallization.

The excellent soft magnetic properties of amorphous (a-) CoFeSiB films make it suited for use in the yoke of granular-in-gap sensors, but only if their thermal stability can be improved. To this end, this study investigated the effects of adding small amounts of other metals on the magnetic and structural properties of a-CoFeSiB films. It was found that adding metals with relatively large atomic radii is an effective way to increase thermal stability, with both Ta and Hf showing good thermal stability after annealing at temperatures of 473 to 573 K. Indeed, a -(CoFeSiB)96.2Hf3.8film was found to maintain its initial coercivity of 0.2 Oe without very little decrease in magnetization after annealing at 623 K. Furthermore, even after annealing at 673 K a -(CoFeSiB)93.0Hf7.0film still had a relatively low coercivity of approximately 0.5 Oe.

Macrospin simulations are performed to model the magnetization switching driven by the combined action of electric-field and spin-polarized electric current (spin-transfer torque; STT) in MgO/CoFeB based magnetic tunnel junctions with interfacial perpendicular magnetic anisotropy. The results indicate that at low current case, the free layer magnetization shows a fast toggle-like switching, the final parallel or antiparallel magnetization state is determined by the electric-field effect, and the STT just helps or resists it to reach the final state depending on the current direction. However, with the increase of current strength, the contribution of STT effect gradually increases, which eventually achieves a deterministic magnetization switching state. Simulations further demonstrate that by appropriately tuning the parameters of applied electric-field and current the power consumption can be easily reduced by two orders of magnitude.

The magnetic properties of Nd/Ni80Fe20 (Py) bilayer films with different layer thicknesses were investigated. Hysteresis loops were measured at different temperatures, an exchange bias effect is found at low temperature due to the Nd/Py interface. By fitting the temperature dependence of the saturation magnetization, the Néel point of Nd layer is obtained. Ferromagnetic resonance (FMR) experiments were carried out at room temperature as a function of Nd layer thickness. The damping parameter obtained by theoretical fitting of the FMR linewidth shows an increasing trend with increasing Nd layer thickness, which is taken as an indication of the spin pumping effect.

We propose a crawling and drilling microrobot actuated by an external precessional magnetic field (EPMF) to effectively unclog obstructed blood vessels. Conventional crawling microrobots can only generate crawling motions using an external oscillating magnetic field. The proposed microrobot can generate navigating (crawling) and drilling motions selectively or simultaneously by controlling the EPMFs. We prototyped the proposed microrobot, and conducted several experiments to verify the efficacy of the crawling and drilling ability of the microrobot in a tubular environment.

We investigate magnetization processes expected for thin films by using a two-dimensional Ginzburg-Landau type model augmented by a long-range dipole-dipole interaction. For homogeneous systems without any extrinsic effects, we find several persisting structures with opposite magnetization in almost saturated magnetic states. They take relatively stable forms, typically, a line, a honeycomb, and so on, which originate from the global connectivity of the initial maze in the sense of percolation. For systems with square-like grain structures, on the other hand, we find maze-like structures that are patterned beyond grain boundaries for zero field. For saturated fields, we again recognize the line-type structure in some grains but do not recognize it in the other grains, strongly depending on the initial pattern in each grain. When we decrease the field, such distinctions are maintained and the latter grains keep their magnetization down to some critical field. The result indicates the duality of the so-called magnetic separability as well as a mechanism of finite coercive fields in the presence of the grain boundaries.

This paper deals with a particular magnetic nondestructive technique applied to the control of the position of the steel blades in rotating parts of turbines and engines. The working principle is based on a bridge of four identical magneto-resistivesensors. One sensor is placed near the blades, and the change in magnetic field produced by a permanent magnet and deviated by the change in position of the blade is detected by the sensor bridge. The position of the sensor is indicated, via dedicated FEM simulations, in order to have high sensitivity to the position change and high output signal. The accuracy and effectiveness of the proposed method are shown by experimental tests carried out in our laboratories. In particular, the tests indicate that the proposed magnetic nondestructive technique can be used in an almost large velocity range, and for quite different values of blade tip. The method seems also promising for the detection of blade vibrations.

The magnetic and magnetocaloric (MC) properties of melt-spun ribbons of the Dy3Co intermetallic compound were investigated. Samples were fabricated in an Ar environment using a homemade melt spinner system at a linear speed of the rotating copper wheel of 40 ms−1. X-ray diffraction analysis shows that ribbons crystallize into a single-phase with the Fe3C-type orthorhombic crystal structure. The M(T) curve measured at 5 mT reveals the occurrence of a transition at 32 K from a first to a second antiferromagnetic(AFM) state and an AFM-to-paramagnetic transition at TN=43 K. Furthermore, a metamagnetictransition is observed below TN, but the magnetization change ΔM is well below the one reported for bulk alloys. Below 12 K, large inverse MC effect and hysteresis losses are observed. This behavior is related to the metamagnetictransition. For a magnetic field change of 5 T (2 T) applied along the ribbon length, the produced ribbons show a peak value of the magnetic entropy change ΔSMpeak of −6.5 (− 2.1) Jkg−1K−1 occurring close to TN with a full-width at half-maximum δTFWHM of 53 (37) K, and refrigerant capacity RC = 364 (83) Jkg−1 (estimated from the product |ΔSMpeak| × δTFWHM).